Due to increasing consciousness for environmental problems, much attention is paid to composites. Applications of carbon fiber as reinforced fiber for composites are spreading in various kinds of fields, and still higher performance is significantly required. Increasing tensile strength of carbon fiber contributes to weight reduction of components such as pressure vessels and, therefore, further increase in tensile strength thereof is an important issue.
In a brittle material such as a carbon fiber, tensile strength of the carbon fiber can be increased by reducing the flaw size of the carbon fiber or increasing the fracture toughness thereof according to Griffith's equation. Particularly, improvement in the fracture toughness of a carbon fiber is effective in that the tensile strength of the carbon fiber can be increased without depending on the state of the flaw size of the carbon fiber (WO 97/45576). Additionally, improvement in the fracture toughness of a carbon fiber is also effective in that tensile strength of a carbon fiber-reinforced composite obtained using the carbon fiber can be efficiently increased.
Until today, as methods of improving tensile strength and modulus of carbon fibers, there have been proposed methods in which an oxidation temperature is increased by using a plurality of ovens different in temperature in an oxidation process and methods in which, in an oxidation oven formed by a plurality of ovens, a precursor fiber for a carbon fiber having passed through each of the ovens is extended according to the density thereof (Japanese Unexamined Patent Application Publication No. S58-163729, Japanese Unexamined Patent Application Publication No. H06-294020, Japanese Unexamined Patent Application Publication No. S62-257422 and Japanese Unexamined Patent Application Publication No. 2013-23778). Additionally, there is a proposed method in which temperature control is performed by using two to three temperature control regions in an oxidation process to make difference in temperature between the regions (Japanese Unexamined Patent Application Publication No. 2012-82541).
Furthermore, techniques to increase torsional modulus of carbon fibers to improve compressive strength thereof are known (Japanese Unexamined Patent Application Publication No. H09-170170, Japanese Unexamined Patent Application Publication No. H05-214614 and Japanese Unexamined Patent Application Publication No. 2013-202803). In investigating the compressive strength of a single-fiber, a carbon fiber single-fiber loop test has been used hitherto (Japanese Unexamined Patent Application Publication No. H09-170170 and Japanese Unexamined Patent Application Publication No. 2014-185402). In Japanese Unexamined Patent Application Publication No. 2014-185402, a high compressive fracture strain has been obtained by using a carbon fiber having low tensile modulus and, in Japanese Unexamined Patent Application Publication No. H09-170170, the compressive strength of a carbon fiber has been increased by using an ion implantation technique. However, those techniques have not been sufficient to increase the tensile strength of the carbon fibers.
There are known techniques that control a single-fiber strength distribution of a short gauge length region of a carbon fiber to improve tensile modulus and open-hole tensile strength of a carbon fiber-reinforced composite (Japanese Unexamined Patent Application Publication No. 2014-159564 and Japanese Unexamined Patent Application Publication No. 2014-159664).
It is important to increase the fracture toughness of a carbon fiber and, to do so, it is essentially important to control the minute structure of the carbon fiber. The proposal of WO 97/45576 controls a silicone oil agent, a single-fiber fineness, and differences between skin-core structures to merely improve physical properties by controlling surface flaws or controlling a minute structure distribution, and does not intend improvement in the minute structure itself.
In Japanese Unexamined Patent Application Publication No. S58-163729, two to three temperature control regions are used in an oxidation process and treatment is performed at a temperature as high as possible in each region. However, the treatment requires as long as 44 to 60 minutes. In Japanese Unexamined Patent Application Publication No. H06-294020, short-time oxidation is performed by using two to three temperature control regions in an oxidation process and increasing heat treatment time in a high-temperature region and, accordingly, oxidation time at high temperature becomes long. Japanese Unexamined Patent Application Publication No. S62-257422 requires three to six ovens to set a plurality of stages for stretching levels in an oxidation oven or reduce oxidation time, but has not achieved satisfactory control of the minute structure of a carbon fiber. Japanese Unexamined Patent Application Publication No. 2013-23778 performs heat treatment for 10 to 120 seconds at 280 to 400° C. after setting a fiber specific gravity during an oxidation process to 1.27 or more. However, control of the minute structure of a carbon fiber has not been made satisfactorily only by the temperature increase in just a final stage of the process. Japanese Unexamined Patent Application Publication No. 2012-82541 controls so that the specific gravity of an oxidated thread after a first oxidation oven is 1.27 or more, and has not satisfactorily achieved minute structure control.
It is difficult to uniformly compare the torsional modulus of a carbon fiber in Japanese Unexamined Patent Application Publication No. H09-170170, Japanese Unexamined Patent Application Publication No. H05-214614 and Japanese Unexamined Patent Application Publication No. 2013-202803 with shear modulus described later, but the following things can be said about the torsional modulus therein. Japanese Unexamined Patent Application Publication No. H09-170170 and Japanese Unexamined Patent Application Publication No. H05-214614 use ion implantation and electron beam irradiation to increase the torsional modulus of a carbon fiber. The obtained carbon fiber contains lattice defects due to covalent bond cleavage and realignment. Thus, the shear modulus of the carbon fiber becomes unsatisfactory, and association with the tensile strength of the carbon fiber is also not considered. Japanese Unexamined Patent Application Publication No. 2013-202803 relates to a carbon fiber that is expected to exhibit physical properties equivalent to a carbon fiber having usual single-fiber fineness, although large in single-fiber fineness. Specifically, a carbon fiber having a shear modulus of 4 GPa or more is disclosed, but has never reached any satisfactory level.
Japanese Unexamined Patent Application Publication No. H09-170170 and Japanese Unexamined Patent Application Publication No. 2014-185402 have not been intended to increase the tensile strength of a carbon fiber and, as a matter of fact, the tensile strength of a carbon fiber determined by its loop shape is not high.
Japanese Unexamined Patent Application Publication No. 2014-159564 has improved open-hole tensile strength by controlling the single-fiber strength distribution of the short gauge length region of a carbon fiber, but has some room for improvement in terms of achieving balance with tensile strength of resin-impregnated strands. Japanese Unexamined Patent Application Publication No. 2014-159664 controls the single-fiber strength distribution of the short gauge length region of a carbon fiber by narrowing the single-fiber diameter of the carbon fiber so that flaws are reduced. There is still some room for improvement to efficiently improve tensile modulus and open-hole tensile strength of carbon fiber-reinforced composites.
It could therefore be helpful to provide a carbon fiber (a bundle of carbon fibers) from which a carbon fiber-reinforced composite having high tensile strength can be obtained and a method of manufacturing the same.